Secondary radar systems are used in the internationally standardized Air Traffic Control Radar Beacon System (ATCRBS) and in military Identification, Friend or Foe (IFF) systems for the advantages offered in providing surveillance radars with much stronger target signal returns, reduced ground clutter and weather return problems, positive identification of targets, and for the capability of exchanging data between an interrogating station and a responding station. In ATCRBS, for example, a ground-based interrogating radar repeatedly transmits, during azimuth scans of its antenna, either or both, identity and altitude interrogation signals. Any transponder equipped aircraft receiving the interrogation signals responds appropriately by transmitting its identification code or the altitude at which it is flying.
The transponder must respond only to interrogation signals originating in the main lobe of the interrogating radar beam, otherwise the angular resolution of the interrogating radar will be seriously degraded. The Sidelobe Suppression System (SLS) is a method in widespread use in conjunction with ATCRBS for inhibiting transponder responses to side lobe interrogations.
In the SLS, a first pulse P.sub.1 of an interrogation signal is transmitted by the scanning directional antenna of the interrogating radar. At a fixed time after transmission of pulse P.sub.1, a control pulse P.sub.2 is transmitted by an omnidirectional antenna at a power level that is less than the power level of the main beam of the directional antenna but that is greater than the power level of all side lobes of the directional antenna. After transmission of pulse P.sub.2, a third pulse P.sub.3 is transmitted by the directional antenna. The spacing between pulses P.sub.1 and P.sub.3 determines the type of interrogation, i.e., whether information on identity or on altitude is sought. The amplitudes of pulses P.sub.1 and P.sub.2 are compared by the transponder and unless the amplitude of P.sub.1 exceeds the amplitude of P.sub.2 by a specified amount, typically 9 dB, a reply by the transponder to the interrogation is suppressed.
Further details of ATCRBS and the side lobe suppression system used therein are given in the publication "Radar Handbook", M. I. Skolnik, ed., pp. 38-1 ff., McGraw-Hill Book Co., 1970.
Certain military IFF systems use an Interrogation Sidelobe Inhibit (ISLI) feature together with a monopulse antenna for preventing undesired responses by transponders to interrogation signals. The radiating elements for one plane of the antenna, say the azimuth plane, are fed in phase with a reference pulse P.sub.ref producing a sum beam azimuth pattern. Then the radiating elements lying on opposite sides of the center of the azimuth plane of the antenna are fed with equal amplitude control pulses I.sub.1 in phase opposition, producing a difference beam antenna pattern. The difference beam antenna pattern is superimposed upon the sum beam antenna pattern in space so that the amplitude of pulse I.sub.1 will exceed the amplitude of pulse P.sub.ref at all azimuth bearings except those lying between the crossover points of the sum and difference antenna patterns. As in SLS, the peak amplitudes of the P.sub.ref and I.sub.1 pulses are compared at the transponder to determine whether the interrogation signal originated in the main beam or in a side lobe of the sum antenna pattern of the interrogating radar.
Pulse I.sub.1 can be transmitted at a higher power level to narrow the portion of the main lobe of the sum beam pattern lying between the crossover points of the sum and difference antenna patterns. Such a reduction of the width of the beam of the interrogating radar to which a transponder will reply is referred to as synthetic beam sharpening.
It is an object of the present invention to provide a method for inhibiting responses by a transponder to side lobe interrogations that will the reduce the effects of noise in the amplitude comparison process which determines whether the transponder will reply to an interrogation signal.
It is another object of the invention to provide a method applicable to IFF systems using monopulse antennas that permits synthetic beam sharpening in an amount controllable by the interrogating radar without requiring an increase in the power level of the control pulses transmitted therein.